linux-kernel - Re: [PATCH RFC 10/35] mm/hugetlb: cleanup hugetlb_folio_init_tail

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Message-ID: <dbd2ec55-0e7f-407a-a8bd-e1ac83ac2a0a@redhat.com>
Date: Mon, 25 Aug 2025 18:23:48 +0200
From: David Hildenbrand <david@...hat.com>
To: Mike Rapoport <rppt@...nel.org>
Cc: Mika Penttilä <mpenttil@...hat.com>,
 linux-kernel@...r.kernel.org, Alexander Potapenko <glider@...gle.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Brendan Jackman <jackmanb@...gle.com>, Christoph Lameter <cl@...two.org>,
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 dri-devel@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
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 Jason Gunthorpe <jgg@...dia.com>, Jens Axboe <axboe@...nel.dk>,
 Johannes Weiner <hannes@...xchg.org>, John Hubbard <jhubbard@...dia.com>,
 kasan-dev@...glegroups.com, kvm@...r.kernel.org,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>, linux-arm-kernel@...s.com,
 linux-arm-kernel@...ts.infradead.org, linux-crypto@...r.kernel.org,
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Subject: Re: [PATCH RFC 10/35] mm/hugetlb: cleanup
 hugetlb_folio_init_tail_vmemmap()

>   
>> We should do something like:
>>
>> diff --git a/mm/memblock.c b/mm/memblock.c
>> index 154f1d73b61f2..ed4c563d72c32 100644
>> --- a/mm/memblock.c
>> +++ b/mm/memblock.c
>> @@ -1091,13 +1091,16 @@ int __init_memblock memblock_clear_nomap(phys_addr_t base, phys_addr_t size)
>>   /**
>>    * memblock_reserved_mark_noinit - Mark a reserved memory region with flag
>> - * MEMBLOCK_RSRV_NOINIT which results in the struct pages not being initialized
>> - * for this region.
>> + * MEMBLOCK_RSRV_NOINIT which allows for the "struct pages" corresponding
>> + * to this region not getting initialized, because the caller will take
>> + * care of it.
>>    * @base: the base phys addr of the region
>>    * @size: the size of the region
>>    *
>> - * struct pages will not be initialized for reserved memory regions marked with
>> - * %MEMBLOCK_RSRV_NOINIT.
>> + * "struct pages" will not be initialized for reserved memory regions marked
>> + * with %MEMBLOCK_RSRV_NOINIT if this function is called before initialization
>> + * code runs. Without CONFIG_DEFERRED_STRUCT_PAGE_INIT, it is more likely
>> + * that this function is not effective.
>>    *
>>    * Return: 0 on success, -errno on failure.
>>    */
> 
> I have a different version :)
>   
> diff --git a/include/linux/memblock.h b/include/linux/memblock.h
> index b96746376e17..d20d091c6343 100644
> --- a/include/linux/memblock.h
> +++ b/include/linux/memblock.h
> @@ -40,8 +40,9 @@ extern unsigned long long max_possible_pfn;
>    * via a driver, and never indicated in the firmware-provided memory map as
>    * system RAM. This corresponds to IORESOURCE_SYSRAM_DRIVER_MANAGED in the
>    * kernel resource tree.
> - * @MEMBLOCK_RSRV_NOINIT: memory region for which struct pages are
> - * not initialized (only for reserved regions).
> + * @MEMBLOCK_RSRV_NOINIT: memory region for which struct pages don't have
> + * PG_Reserved set and are completely not initialized when
> + * %CONFIG_DEFERRED_STRUCT_PAGE_INIT is enabled (only for reserved regions).
>    * @MEMBLOCK_RSRV_KERN: memory region that is reserved for kernel use,
>    * either explictitly with memblock_reserve_kern() or via memblock
>    * allocation APIs. All memblock allocations set this flag.
> diff --git a/mm/memblock.c b/mm/memblock.c
> index 154f1d73b61f..02de5ffb085b 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -1091,13 +1091,15 @@ int __init_memblock memblock_clear_nomap(phys_addr_t base, phys_addr_t size)
>   
>   /**
>    * memblock_reserved_mark_noinit - Mark a reserved memory region with flag
> - * MEMBLOCK_RSRV_NOINIT which results in the struct pages not being initialized
> - * for this region.
> + * MEMBLOCK_RSRV_NOINIT
> + *
>    * @base: the base phys addr of the region
>    * @size: the size of the region
>    *
> - * struct pages will not be initialized for reserved memory regions marked with
> - * %MEMBLOCK_RSRV_NOINIT.
> + * The struct pages for the reserved regions marked %MEMBLOCK_RSRV_NOINIT will
> + * not have %PG_Reserved flag set.
> + * When %CONFIG_DEFERRED_STRUCT_PAGE_INIT is enabled, setting this flags also
> + * completly bypasses the initialization of struct pages for this region.

s/completly/completely.

I don't quite understand the interaction with PG_Reserved and why 
anybody using this function should care.

So maybe you can rephrase in a way that is easier to digest, and rather 
focuses on what callers of this function are supposed to do vs. have the 
liberty of not doing?

-- 
Cheers

David / dhildenb